1. Field of the Invention
The present invention relates to a distance measuring device and more particularly to the distance measuring device for a photographic camera.
2. Description of the Prior Art
A wide variety of distance measuring devices for measuring the distance from the device to a target object have been proposed. Such distance measuring devices may be divided mainly into so-called passive systems or active systems. In active systems, a projector in the camera scans the object space with a beam of light and the reflected light is detected to find the distance. In a passive system, light from an object is converted into an electrical signal which is then processed to find the distance, without projecting any special measuring light onto the object.
In one example of an active distance measuring device, the distance measuring operation is carried out by making a so-called zone mark that indicates where the projecting beam is searching (hereinafter referred to as "the scanning area") coincide with the image of the object on the focusing screen.
Such conventional distance measuring devices assure highly accurate distance measurements when applied to situations where it is easy to establish coincidence between the object image and the zone mark. This is so, for example, when the object is intensely illuminated or very bright, such as when the distance measurement is performed in sunlight. Under these circumstances, the object can be accurately caught. However, when the coincidence between the object image and the zone mark is difficult to achieve, accurate distance determinations cannot be made. This occurs, for example, when the object appears in a dark location and is itself dim, or when the camera is in the self-timer exposure mode. In other words, when the object is in a dark location and has a low reflectance, the object image in the finder is dark and the zone mark also is dark, so that it becomes very difficult to establish coincidence of the zone mark with the object image. Often, because the beam from the projector is turned away from the object, it is impossible to make sure of accurate distance measurements.
Other active distance measuring devices exhibit similar problems during night photography because the object whose distance is to be measured cannot be accurately perceived.
Also, in the latter case, where the photographer uses a self-timer and makes himself the object, the photographer can hardly make himself the object to coincide with the zone mark. Hence, it has been the common practice to preset the zone mark by presuming a position which the object is to occupy during a shot. However, such an arrangement exhibits a drawback in that accurate distance measurement cannot be assured because the object often assumes a position outside the target area of the projector beam.